It is well known that hydrogen peroxide, such as in sodium percarbonate and sodium perborate, is formulated in a powdery detergent composition (synthetic detergent for family use) as a bleach component. Hydrogen peroxide, such as in sodium percarbonate and sodium perborate, dissolves in laundry to perform bleaching action.
In this case, the solubility rate of sodium perborate at a low temperature is small, so it is not preferable for a bleach component formulated in a detergent in Japan, where cold water or tepid water is used in laundry. On the other hand, the solubility rate of sodium percarbonate at a low temperature is high, and the bleaching effect is sufficiently performed, so the need for it has increased rapidly in recent years.
However, sodium percarbonate is relatively sensitive to water content and is easy dissolved at room temperature by water content in a detergent composition or by moisture or water content in the air. Further, there is a case that substances such as zeolites or enzymes, which accelerate the dissolution of sodium percarbonate, are contained in detergent composition, and it is dissolved by contacting with the substances.
Thus, until this time, several methods in which the dissolution of sodium percarbonate is prevented or prohibited to obtain stabilized sodium percarbonate have been suggested. For example, there are methods in which stability is performed by adding a stabilizer such as sodium metasilicate, a magnesium compound and a chelating agent at the time of crystallization; by adding additives such as a binder or phosphates at the time of wet granulation; and by coating a dried surface of sodium percarbonate.
Among these, the third method (that is, the method in which a particle of sodium percarbonate is coated by several kinds of coating agents) is the most powerful method.
As the coating agent, a salt of alkaline earth metal or a mixture of sodium carbonate and sodium sulfate, etc., are suggested.
As the method in which a particle is coated with an alkaline earth metal salt, the method described in Japanese Patent Publication Sho 57-7081 is known. This method is the method in which the surface of sodium percarbonate is catalytically reacted with an alkaline earth metal salt solution, and a membrane comprising an alkaline earth metal carbonate is formed on the surface of the sodium percarbonate. This method certainly can raise somewhat the stability of sodium percarbonate, but there are two problems as described below.
One problem is that when sodium carbonate in sodium percarbonate is reacted with an alkaline earth metal salt, the hydrogen peroxide which is liberated decomposes at the time of drying, so the effective component of sodium percarbonate decreases.
Another problem is that by the production of an insoluble alkaline earth metal carbonate, the solubility rate becomes extremely small, so it is difficult to use.
On the other hand, in case of the method in which a particle is coated with a mixed salt of sodium carbonate and sodium percarbonate or sodium sulfonate (Japanese Patent Publication Sho 58-24361), the solubility is relatively good, but the stability of the formulation with detergent does not reach a practical level, although it improves somewhat as compared with uncoated sodium percarbonate.